Recently, a distributed antenna system (DAS) which uses distributed antennas within a cell has been discussed. FIG. 1 is a diagram illustrating comparison between a centralized antenna system (CAS) and a DAS.
As illustrated in FIG. 1, the DAS is a network of antennas connected to a common source through a transmission medium and spatially spaced apart from one another. The DAS can extend coverage and enhance energy efficiency. As an antenna is located near a mobile station, a shade zone can be removed, and stable connection between the antenna and the mobile station can be performed. Also, path loss or propagation loss is reduced, and power efficiency is increased as connection quality between a base station and a mobile station is improved.
FIG. 2 is a diagram illustrating an example of a method of using and configuring a DAS. In addition to the advantage of coverage extension, the DAS has an advantageous in that it increases capacity. FIG. 2(a) illustrates that the same signal from all antennas is repeatedly transmitted, and FIG. 2(b) illustrates that independent data is transmitted from each of all antennas. Theoretically, the DAS increases average link capacity as much as two times, approximately, in accordance with simple selection of antennas. In order to more improve system capacity, the DAS should support independent data transmission from each of the antennas.
The DAS may be used together with another technique such as a relay and femto cell. FIG. 3 is a diagram illustrating an example of configuring a DAS from a CAS having a femto cell.
In spite of the fact that the DAS has been discussed as above, a method how to acquire cell synchronization from a DAS in a mobile station and maintain the cell synchronization has not been developed.